The ability to transfer microscopic patterns onto semiconductor substrates is essential to device fabrication in the microelectronics industry. The most common method of pattern transfer involves deposition of an organic photoresist onto the substrate. The desired pattern is then transferred to the photoresist by passing light of an appropriate wavelength through a mask with the desired pattern. Once the photoresist is exposed, a portion of it is selectively removed to complete transfer of the pattern. In the positive photoresists typically used in current processes, the exposed portion of the photoresist is selectively removed by a solvent.
After transfer of the pattern, the photoresist remaining on the substrate acts as a mask to allow selective processing of the underlying substrate. This subsequent processing can include implantation of dopant atoms or etching of the underlying substrate material.
Once this processing is complete, the remaining photoresist material on the substrate must be completely removed prior to beginning the next series of process steps. Several techniques are currently used for this removal. Aqueous mixtures of sulfuric acid and hydrogen peroxide at temperatures of 80.degree. C.-150.degree. C. effectively remove most post process photoresist residues. When the underlying substrate includes metal lines, however, these mixtures are not suitable as they will damage the metal lines. Various organic solvents, such as N-methyl-pyrrolidone (NMP), may be used to remove the undesired photoresist without harming the metal lines. However, these solvents tend to pose both safety and health hazards. Also, solvents are generally expensive and incur high disposal costs. Oxygen ashers, which use microwave energy to create an oxygen plasma to remove the photoresist, provide an alternative' method for photoresist removal. However, this method typically does not leave the surface in a suitably clean state for subsequent processing, thus requiring an additional step to finish the substrate cleaning. Additionally, the harsh environment within the asher has the potential to damage the substrate.
Mixtures of sulfuric acid and ozone have also been employed for the removal of photoresist. Sulfuric acid and ozone are used to strip photoresist in a wet bench with a recirculation system for the sulfuric acid. The resist is partially oxidized by the sulfuric acid and removed from the substrate into solution thereby contaminating the sulfuric acid. Ozone is bubbled through the sulfuric acid to finish the breakdown of the photoresist in order to clean the sulfuric acid so that the sulfuric acid can be reused.
An alternative to the above photoresist removal methods is the use of ozone as the primary chemical agent for photoresist removal. Although gas phase ozone may be used for removal of photoresist and other organics, ozonated water has several advantages over gas phase ozone when liquid water may be used in the processing environment. In a completely gas phase process, the ozone must react with the organic contaminants until the resulting molecules are volatile in the gas phase. In aqueous solution, however, it is only necessary to react ozone with the organics until the resulting molecules are soluble and can be transported away by diffusion or by mechanical rinsing. The latter task is generally easier to accomplish, leading to more effective organic removal with solutions of ozone in water.
Use of ozonated water is an attractive method of photoresist removal as it eliminates many of the problems of the traditional photoresist removal methods. Solutions of ozone in water are easy and relatively inexpensive to make at the point of use. Because the ozone in the water eventually breaks down into molecular oxygen, ozonated water does not require as many of the special disposal techniques needed for the other liquid phase removal methods. It is also suitable as a one step clean and it is compatible with all of the substrates commonly exposed during microelectronic fabrication.
In a more general sense, ozonated water is suitable for a variety of organic removal applications of which photoresist removal is but one example. Solutions of ozone in water will react with a wide variety of organic materials. Generally, ozone will react directly with molecules containing carbon-carbon double bonds, carbon-carbon triple bonds, carbon-oxygen bonds, or carbon-nitrogen bonds. Direct reaction implies that the ozone does not pass through a reaction intermediate when reacting with the above molecules. Direct ozone reactions with compounds containing only carbon and hydrogen or compounds containing aromatic rings, such as benzene and phenol, are relatively slow. Ozone reactions with these compounds usually involve an indirect reaction mechanism, implying that the ozone molecule is first converted into another species prior to reaction, such as an OH radical. Common photoresists generally contain a wide variety of functional groups, including carbon-carbon multiple bonds and carbon-heteratom bonds. As a result, ozone reacts quickly with photoresist via direct reactions.
U.S. Pat. No. 5,464,480 by Matthews details a process for increasing the concentration of ozone for use in removing thick organic layers such as photoresist. The increased ozone concentrations are achieved by reducing the temperature of the water during the dissolution of the ozone gas into the water, thus increasing the solubility of the ozone gas in the water.
While ozonated water will not damage typical substrates, it is not universally effective for removing photoresist or other organic materials under all conditions. The presence of exposed metal lines such as aluminum or copper lines in the underlying substrate, has been observed to inhibit the effectiveness of ozone in aqueous solution as a photoresist removal agent.
It is a goal of the present invention to provide a method for the enhanced removal of photoresist and other organic materials from the surface of a substrate for use in an electronic device. It is a further goal of the present invention to provide a method for the removal of photoresist and other organic materials from the surface of a substrate in the presence of metals such as copper and aluminum on the surface of the substrate.